Method for matching colors

ABSTRACT

A method for matching colors includes providing a first source of a first color and a second source of a second color; measuring a set of spectral values for first color stimuli associated with the first color; calculating a first set of tristimulus values; defining a first set of human observer color matching functions; calculating a first set of corrected tristimulus values from the first set of spectral values using the first set of human observer color matching functions; measuring a second set of spectral values for second color stimuli associated with the second color; calculating a second set of tristimulus values; defining a second set of human observer color matching functions; calculating a second set of corrected tristimulus values from the second set of spectral values; determining a difference between the first and second set of corrected tristimulus values; and adjusting one of the sources to reduce the difference.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. ______ (Attorney Docket No. 95734/NAB), filed herewith,entitled A METHOD FOR CONVERTING DIGITAL COLOR IMAGES, by ChristopherEdge; U.S. patent application Ser. No. ______ (Attorney Docket No.95776/NAB), filed herewith, entitled METHOD FOR CHARACTERIZING THE COLORRESPONSE OF AN IMAGING DEVICE, by Christopher Edge; U.S. patentapplication Ser. No. ______ (Attorney Docket No. 95777/NAB), filedherewith, entitled METHOD FOR REPRODUCING AN IMAGE ON AN IMAGING DEVICE,by Christopher Edge; U.S. patent application Ser. No. ______ (AttorneyDocket No. 95778/NAB), filed herewith, entitled METHOD AND APPARATUS FORMEASURING COLORS, by Christopher Edge; and U.S. patent application Ser.No. ______ (Attorney Docket No. 95779/NAB), filed herewith, entitledMETHOD FOR MATCHING COLORS BETWEEN TWO SYSTEMS, by Christopher Edge; thedisclosures of which are incorporated herein.

FIELD OF THE INVENTION

The present invention relates to the field of color metrology as appliedto color management. In particular, it relates to adjusting humanobserver functions in terms of the location in color space.

BACKGROUND OF THE INVENTION

Human observer functions x(λ) y(λ) z(λ) are response functions used tocharacterize colors perceived by the human eye. The existing CommissionInternationale de l'Éclairage (CIE) and International Organization forStandardization (ISO) definitions of the human observer functions arethe basis for color management systems and for standard profile formatssuch as those defined by the International Color Consortium (ICC). Thefact that the ICC has been embraced and incorporated into operatingsystems such as Apple™ OS X and applications such as Adobe™ PhotoShop™is evidence that the standard is reasonably useful and effective forensuring or at least improving the quality of color appearance ofimages. The paradigm of conversions from “source” to “destination”implies preserving quality and/or appearance of digital color images.

Standards committees such as IDEAlliance have defined very clearmeasured color target values and tolerances using (CIEXYZ) and (CIELAB)in order to ensure that the visual appearance of hard copy proofingsystems from multiple venders are consistent for a given digital inputimage file. Assessments of prints generated by multiple venders usingthese standards have been performed on numerous occasions with greatsuccess both within committee meetings and at public events to showcasethe efficacy of these standards. In most cases, venders make use of theICC standard or similar colorimetrically-based formats in order toensure correct output of printed images.

Although existing CIE and ISO standards appear to work well for multipleprints viewed under the same illumination, achieving the same level ofstandardization for soft proofing has proven to be more challenging. Anexample of this reality is the fact that the calculations for CIELAB inthe IDEAlliance certification of soft proofing systems are normalized tothe white point of the display rather than to D50. The reason for thisnon-standard calculation of CIELAB is the widespread practice ofadjusting the white point of a display in order to match visually thewhite balance of the D50 illumination used to view the correspondingprint. In theory, it should be adequate to measure the white balance ofthe illumination and merely ensure that white point of the display isadjusted to match the illumination according to measured values.

In reality, an offset to the measured white is often applied in order toensure a good visual match between display and print. However, it hasbeen observed that it is difficult to ensure both accurate appearance ofwhite/gray balance and accurate appearance of other critical regions ofcolor such as skin tones. If there exists an error in the standard, onewould expect a simple correction would be sufficient.

The need for a slight modification or offset to the white point appearsto have some validity as indicated by scientific studies that have beenperformed in the area of color matching functions determined via the“Maxwell method,” i.e. the matching of whites, vs. the more common“saturation method,” which determines color matching functions based onmatching saturated colors. For example, in section 5.6.6 of ColorScience: Concepts and Methods, Wyscecki summarizes the work done in thearea of comparing the saturation and Maxwell methods. At the end of thesection, Wyscecki states, “The deviations are appropriately termedfailure of the additivity law of color matching in a bipartite field.Suggestions have been made with regard to possible causes of thesefailures. They include chromatic adaptation (Crawford, 1965), Maxwellspot (Palmer, 1980), and interactions or linkages between different conemechanisms (Ingling and Drum, 1973), but further work is obviouslyneeded to resolve the conundrum.”

Likewise, in a series of papers I-VI entitled “Toward a more accurateand extensible colorimetry,” Thornton describes various experimentsinvolving the matching of whites. Converting his data to units of CIELABindicate a disagreement between observed matches of different whitespectra and predicted matches of up to 25 ΔE as calculated using thestandard human observer. In a similar study published in 1993 by Northand Fairchild, “Measuring Color-Matching Functions. Part II” (1993,North and Fairchild) Thornton's observations appeared to be confirmed,although the analysis of the results was performed based on the model ofthe deviate human observer, i.e. was explained by the differencesbetween individual observers rather than interpreted to imply acorrection to existing standard human observers functions.

Most recently at the Society for Imaging Science and Technology (IS&T)Color Imaging Conference 16 in Portland, Oreg., a presentation, “ColorVision and More Comprehensive Color Appearance Models” was given byHirohisa Yaguchi, Chiba University (Japan). In this presentation,Yaguchi-san showed plots of color matching functions obtained using hisown eyes on his color matching apparatus using first the saturation andthen the Maxwell method. He pointed out that the plots containeddifferences that were not insignificant, although no further explanationwas given.

Virtual proofing systems are required to display an accurate matchbetween display and hard copy with little effort from the user. If oneaccounts for metamerism and for errors in conventional XYZ calculations,one finds that the overall hues and white balance are reasonable. Forcritical color requirements, however, there can exist unacceptablevisual differences between display and hard copy in skin tones (forexample) that equate to several percentage points of change in magentadot gain. Although this error is small, it is enough to be problematicfor critical work.

Therefore there remains a need for an improvement to conventional XYZcalculations. It is noted that the “deviant observer” described byFairchild does vary or modify the x(λ) y(λ) z(λ) functions with size ofthe color observed (characterized as an angle indicating the size of thecone of viewing from the eye to a circle indicating the region of thecolor being observed), yellowing of the lens of the eye due to age,effect of the macula of the eye, etc. These adjustments endeavor toaccount for differences due to size of color and to account for observerto observer differences. However, the particular observer functions fora given size of color and particular individual is the same regardlessof the spectral power distribution (SPD) S(λ) of the color stimulusbeing observed. Reference is made to such observer functions as “static”meaning they do not change with SPD S(λ). By contrast, reference is madeto human observer functions that change or adapt depending on the SPDS(λ) of the particular color being viewed as “non-static” meaning thatthey do vary with S(λ).

If the human observer functions were static with S(λ), pairs of colorswould always match as long as the calculated values of XYZ were nearlythe same, regardless of whether the colors were very saturated or veryneutral. However, if the human observer functions are not static withS(λ), it is quite possible for saturated color pairs to match but forneutral pairs of colors to appear different even though their XYZ's werethe same. By allowing the observer functions to be non-static with S(λ),discrepancies between saturated and neutral regions of color can beresolved, thereby ensuring that pairs of color will always match whentheir XYZ's are similar regardless of their S(λ) characteristics.

The observer functions are the basis for color management systems. Areview how color management systems work in relation to the XYZ valuescalculated using the observer functions, and L*a*b* values derived inturn from XYZ is necessary, with the understanding that any improvementsor changes to the observer functions will correspondingly affect XYZ,L*a*b*, and the functionality and efficacy of color management systemsthat rely on these color values.

Current color management systems such as Apple ColorSync™ convert colorpixel data from source device dependent coordinates to destinationdevice dependent coordinates via a device independent color spacereferred to as the Profile Connecting Space or PCS. The PCS is generallybased upon tristimulus values CIEXYZ or perceptually more uniform colorspaces such as CIELAB which are calculated from CIEXYZ.

The general procedure is to define source colors either as a list of PCSvalues (such as the Pantone™ library) or as a set of device pixel valuessuch as CMYK with an associated profile or characterization. The mostcommon format for such profiles is that of the International ColorConsortium (ICC). In the latter case, the processing engine of thesystem known as the color matching module or method (CMM) converts thedevice dependent values (e.g. CMYK) to an appropriate PCS (e.g. CIELABor CIEXYZ) via interpolation of tables within the ICC profile for thedevice.

In order for these PCS values to demonstrate usefulness, the colormanagement system is usually invoked for the purpose of converting thesePCS values (once they are fetched or calculated) to a device dependentdestination such as an RGB monitor or digital projector. This isperformed via the color matching method (CMM) in conjunction with theICC profile of the destination device. Implicit in the above system isthe assumption that at some point prior to the above conversion, colormeasurements have been performed in order to calculate the PCS values.For individual colors in a library such as Pantone™, this is the onlyeffort that is required. In the case of a complex device requiring anICC profile, multiple color measurements are performed and highresolution look up tables are calculated from that data in order toconstruct a reasonable approximation to the color characteristics of thedevice, for example, to estimate reasonable expected measured values ofL*a*b* or XYZ for any combination of CMYK pixel values. Likewise, forcomplex devices, an inverse table is constructed and stored in the ICCprofile to convert PCS values to the devices codes of the device, e.g.RGB or CMYK.

All the above requires a means for measuring and quantifying color. Thetwo most common methods are spectral measurement and colorimetricmeasurement. Spectral measurement results in spectral data, either inthe form of a reflectance spectrum R(λ) or the spectral powerdistribution (SPD) S(λ). The former is used for reflective materialswhile the latter is used for an emissive device such as a monitor. Sincethe SPD is required to calculate XYZ using the observer functions, theSPD for reflective materials is usually calculated by multiplying R(λ)by the SPD of the illuminant I(λ).

The present invention modifies the behavior of the human observerfunctions as they vary with λ based on changes of location in colorspace of the color being measured. In particular, colors in the regionof white and gray will require somewhat different observer functionsfrom colors that are saturated. These modifications to the calculationof XYZ from S(λ) will affect the characterizations and the conversionsof colors processed by a color management system. In particular, thephysical outcome of this invention will be that images reproduced on adisplay or digital projector using a color management system willmeasure differently from the current systems particularly in regions ofneutral and white.

Furthermore, for systems comprising extremely narrow band primaries,such as a projector using RGB lasers, there will be a significantimprovement in the color reproduction of original color images. Forexample if the original image is displayed on a monitor, converted withthe color management system, and displayed using a projector with RGBlasers, the visual match between the two images will be significantlyimproved particularly in regions of white and gray if the colormanagement system is modified to use the improved human observerfunctions.

SUMMARY OF THE INVENTION

The present invention describes a color processing system that utilizesan improved method of defining the human observer functions. The newcolorimetric definition is based on human observer functions that arenot static, as in the prior art, but rather vary as a function of S(λ),the spectral power distribution of the color stimulus. In particular,the human observer functions are assumed to converge to one set offunctions in regions of saturated color and converge to a different setof functions in regions of near neutrality, with a smooth transitionconnecting these two regions.

The expected impact of the present invention, which has been reduced topractice, is that one would expect a good visual match between systemswith different spectral characteristics using measurement basedcalculations in colorful regions, skin tones, and near neutral imagessuch as black and white photos. In general, the present invention willbe particularly helpful for preserving color appearance on systems thathave significantly narrow band properties of RGB primaries, which istypically the case for systems with large color gamuts, RGB laserprojection systems and OLED displays in particular.

Tests of this invention indicate that if one

-   -   a) assumes that the existing standards for x(λ) y(λ) z(λ) (which        are defined using the saturation method) are indeed valid for        saturated colors;    -   b) determines corrections for ensuring a good match in near        neutral regions of color;    -   c) proceeds to connect smoothly these uncorrected and corrected        resulting values of XYZ as one transitions from saturated to        neutral regions of color; and    -   one will achieve good matches of color between systems with        significantly different spectral properties. This approach        guarantees agreement between the saturated and Maxwell color        matching experiments, and also resolves some of the remaining        issues identified with virtual proofing systems.

Briefly, according to one aspect of the present invention a method forcharacterizing a color in terms of tristimulus values includes providinga source of a color; measuring a set of spectral values for colorstimulus associated with the color using a spectral measurement devicecontrolled by a digital processing system; calculating a first set oftristimulus values from the set of spectral value; defining a set ofhuman observer color matching functions, the set of human observer colormatching functions being functions of the tristimulus values; anddetermining a second set of tristimulus values from the set of spectralvalues using set of human observer color matching functions.

According to one aspect of the present invention a method for matchingcolors includes providing a first source of a first color and a secondsource of a second color; measuring a first set of spectral values forfirst color stimulus associated with the first color with a spectralmeasurement device controlled by a digital processing system;calculating a first set of tristimulus values from the first set ofspectral values; defining a first set of human observer color matchingfunctions, the first set of human observer color matching functionsbeing functions of the first set of tristimulus values of the firstcolor; calculating a first set of corrected tristimulus values from thefirst set of spectral values using the first set of human observer colormatching functions; measuring a second set of spectral values for secondcolor stimulus associated with the second color with a spectralmeasurement device controlled by a digital processing system;calculating a second set of tristimulus values from the second set ofspectral values; defining a second set of human observer color matchingfunctions, the second set of human observer color matching functionsbeing functions of the second set of tristimulus values of the secondcolor; calculating a second set of corrected tristimulus values from thesecond set of spectral values using the second set of human observercolor matching functions; determining a difference between the first setof corrected tristimulus values and the second set of correctedtristimulus values; and adjusting one of the first and the second sourceto reduce the difference.

According to one aspect of the present invention a method for matchingcolors includes comparing the appearance of a first white colorassociated with a first color imaging system and a second white color aassociated with second color imaging system, wherein the tristimulusvalues of the first white color and second white color are similar;empirically determining a fixed correction to the tristimulus values ofthe second white color associated with the second color imaging systemin order to achieve a visual match to the first white color of the firstcolor imaging system; measuring a first set of spectral values for afirst color associated with the first color imaging system; determininga first set of tristimulus values from the first set of spectral values;measuring a second set of spectral values for a second color associatedwith the second color imaging system; determining a second set oftristimulus values from the second set of spectral values; applying acorrection to the tristimulus values of the second color, the correctionbeing a function of the tristimulus values as well as a function of thefixed correction; determining a difference between the tristimulus valueof the first color and the corrected tristimulus value of the secondcolor; and adjusting the second color to reduce the difference.

According to one aspect of the present invention a method for the colorresponse of an imaging device includes reproducing a color on theimaging device based on a set of device color coordinates; measuringspectral values for the reproduced color with a spectral measurementdevice controlled by a digital processing system; calculating a firstset of tristimulus values from the spectral values; defining a set ofhuman observer color matching functions, the set of human observer colormatching functions being functions of the first set of tristimulusvalues of the reproduced color; calculating a second set of tristimulusvalues from the spectral values using the defined set of human observercolor matching functions; and associating the reproduced device colorcoordinate with the second set of tristimulus values.

According to one aspect of the present invention a method forreproducing an image on an imaging device includes measuring a set ofspectral data for each of a plurality of colors generated from a set ofcolor device values with a spectral measurement device controlled by adigital processing system; determining a first set of tristimulus valuescorresponding to each set of spectral data; defining a set of humanobserver color matching functions for each set of tristimulus values,wherein the human observer color matching functions are functions of thetristimulus values; determining a second set of tristimulus values foreach set of spectral data using the human observer color matchingfunctions corresponding to the first set of tristimulus values; creatinga first map from device-dependent coordinates to device-independentcolor coordinates derived from the above sets of color device valuespaired with the associated second set of tristimulus values; creating asecond map from device-independent coordinates to device-dependent colorcoordinates based on the first map; obtaining image data comprisingdevice-independent color coordinates; converting the image datacoordinates into device color coordinates via interpolation of thesecond map; and reproducing the image on the color device using thedevice color coordinates.

According to one aspect of the present invention a color measurementdevice includes an aperture for obtaining light from a color sample tobe measured; a plurality of sets of color filters associated with humanobserver functions at different regions of color space; at least onedetector for measuring the intensity of filtered light; an analog todigital converter for converting voltage signals from at least onedetector to digital values representative of tristimulus valuesassociated with each of the sets of color filters; and a processor tocombine the sets of tristimulus values in order to calculate a final setof tristimulus values, the combining being a function of at least one ofthe sets of tristimulus values.

According to one aspect of the present invention a method for measuringcolors includes obtaining light via an aperture from a color sample tobe measured; providing a plurality of sets of color filters associatedwith human observer functions at different regions of color space;obtaining signals proportional to the intensity of filtered lightthrough the plurality of sets of color filters by means of at least onedetector; using at least one analog to digital converter to convert thesignals from the at least one detector to digital values representativeof tristimulus values associated with each of the sets of color filters;and combining the sets of tristimulus values in order to calculate afinal set of tristimulus values, the combining being a function of atleast one of the sets of tristimulus values.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart of a method for characterizing a color, whereincan be seen that the SPD S(λ) of the color stimulus is used to calculateXYZ which in turn is used to create new observer functions x′(λ) y′(λ)z′(λ). These are used to recalculate new tristimulous value X′Y′Z′ fromthe SPD S(λ).

FIG. 2 shows a similar flowchart for two colors, wherein the resultingtwo sets of corrected tristimulus values are compared and one of the twocolors are adjusted in order to improve the visual match between thecolors.

FIG. 3 shows a flowchart wherein a color association is made betweendevice coordinates and corrected tristimulus X′Y′Z′₀ in order to developa device characterization.

FIG. 4 shows a similar flowchart wherein a device characterization iscreated and then used to create an inverse map in order to convertdevice independent image data to output color coordinates for thepurpose of reproducing the image on the device.

FIG. 5 shows a diagram for a colorimeter wherein two sets of filters,which pertain to the color matching functions derived from thesaturation method and the Maxwell method respectively, are used tocreate the new corrected values of XYZ_(C).

FIG. 6 is a flowchart showing the same concept as above but in the formof a process. Voltages are obtained from light detectors behind filtersthat resemble two sets of color matching functions. An A/D convertsthese signals to digital values and the values are combined to createthe corrected X′Y′Z′.

FIG. 7 shows a second method for matching colors between two systemsbased on the correction to XYZ required between two systems in theregion of white. Standard XYZ calculations of the second system arecorrected as a function of original tristimulus value XYZ₂ and whitecorrection ΔXYZ_(W).

DETAILED DESCRIPTION OF THE INVENTION

The human observer functions x(λ) y(λ) z(λ) are response functions usedto characterize colors perceived by the human eye. The integrated sum ofthese functions with a color stimulus of spectral power distribution(SPD) S(λ) results in three values, XYZ which uniquely identifies acolor. Even if two colors have different SPDs, S₁(λ) and S₂(λ),according to color theory they will be perceived as the same color iftheir values of XYZ are the same.

The term used to describe the color science concept upon which thepresent invention is based is non-static human observer functions(NSHOF). Existing definitions of human observer functions are implicitlystatic with respect to the SPD S(λ) of the color being observed. Thismeans that the same x(λ) y(λ) z(λ) functions are used to calculate thetristimulus values XYZ regardless of the behavior of the SPD S(λ):

{right arrow over (XYZ)}=∫S(λ){right arrow over (xyz)}(λ)dλ  (Eq. 1)

where S(λ) is the spectral power distribution of the stimulus on theeye.

$\begin{matrix}{{\overset{}{xyz}(\lambda)} = \begin{pmatrix}{\overset{\_}{x}(\lambda)} \\{\overset{\_}{y}(\lambda)} \\{\overset{\_}{z}(\lambda)}\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

is the vector of the human observer functions and

$\begin{matrix}{\overset{}{XYZ} = \begin{pmatrix}X \\Y \\Z\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

is the vector of the calculated CIEXYZ values.

Historically the functions x(λ) y(λ) z(λ) have been modified for smallareas of color (known as the 2° observer) vs. larger areas of color(known as the 10° observer). The deviant observer described by Fairchildcan furthermore be adjusted for various ranges of size of color as wellas modified on a per individual basis with regards to lens and macula.However, given a particular individual and size of color being observed,the dependence of x(λ) y(λ) z(λ) on λ do not change as a function of thespecific color being viewed as described by its SPD S(λ).

If one finds that the above static functions work well for saturatedcolors, but finds that there is a consistent correction to x(λ) y(λ)z(λ) that is necessary for colors in near-neutral regions, a reasonableconclusion can be that human observer functions change as a function ofthe SPD S(λ) of the color being observed. If the corrections appearconsistent based on color space location only (for example, neutral vs.saturated region), then it can be assumed that x(λ) y(λ) z(λ) varies asa function of the tristimulus values XYZ derived from S(λ) which can beestimated from the conventional x(λ) y(λ) z(λ) functions. By definition,the x(λ) y(λ) z(λ) human observer functions can no longer be regarded asstatic with regard to location in color space XYZ.

A generalized mathematical framework for describing NSHOF assumes:

{right arrow over (xyz)}(λ)→{right arrow over (xyz)}_(C)(λ, S(λ))→{rightarrow over (xyz)}_(C)(λ, f({right arrow over (XYZ)}))   (Eq. 4)

In other words, the corrected color matching functions x _(C)(λ) y_(C)(λ) z _(C)(λ) may vary as a function of the integrated values of XYZcalculated using the standard x(λ) y(λ) z(λ) functions. If the exactproperties of the generalized correction to the standard x(λ) y(λ) z(λ)functions in regions of near neutral, are not known, but one example ofa specific correction between two systems is known, the above expressioncan be modified to correct for a specific system:

{right arrow over (XYZ)}→{right arrow over (XYZ)}_(C)({right arrow over(XYZ)})   (Eq. 5)

Note that for ease of use with existing the tristimulus values XYZ andL*a*b* infrastructures, these corrections are described in terms ofXYZ_(C) and x _(C)(λ) y _(C)(λ) z _(C)(λ). However, since the humanobserver functions are linear transforms of the cone sensitivities l(λ)m(λ) s(λ), this mathematical description can be performed just as wellin “LMS” space, or any other three component visual color space which isbased upon the original color matching functions r(λ) g(λ) b(λ).Document CIE 15:2004 Appendix B contains a good description of x(λ) y(λ)z(λ) as they relate to r(λ) g(λ) b(λ). Hunt's “The Reproduction ofColor” Chapter 31 contains a good description of the cone sensitivitiesl(λ) m(λ) s(λ) (which he prefers to call ρ(λ) γ(λ) β(λ)) as they relateto x(λ) y(λ) z(λ)). The approach remains the same, which is to definesensitivity functions that pertain to the saturation method of colormatching and modified functions that pertain to the Maxwell method ofcolor matching. The tristimulus values are then calculated from at leastone of the two functions and are used to calculate the combining of thetwo extreme cases. In the case of correcting the cone sensitivities,l(λ) m(λ) s(λ), the integrated LMS cone values are used for determiningthe corrected l(λ) m(λ) s(λ) functions in a manner very similar to themethods described herein for correcting x(λ) y(λ) z(λ) using theintegrated XYZ values. It is also possible to use tristimulus valuesthat differ from the spectral function being corrected. For example,f(XYZ) in equation 4 could also be f(RGB) where the RGB values pertainto a standard RGB space such as Adobe™ RGB 1998. Another example sparewould be “EdgeRGB” described in U.S. Pat. No. 7,554,705 and shown inFIG. 5 of that issued patent.

Note that one advantage of using an RGB space as the tristimulus spacefor calculating corrections to x(λ) y(λ) z(λ) is that estimates ofsaturation vs. neutrality can be more accurate. For example, in the caseof EdgeRGB, the locus in CIE space of values of R, G, or B approaching 0(minimum) with at least one value approach 1.0 (maximum) will be veryclose to the locus in CIE space where the eye is viewing monochromaticlight of wavelength λ, i.e. the region in CIE space of maximum gamut ormaximum saturation.

Example Mathematical Approaches for Calculating NSHOF

1. Linear Combination of Two Static Sets of x(λ) y(λ) z(λ)

A first example defines two sets of human observer functions, onedetermined using the saturation method and one defined using the Maxwellmethod (indicated by subscripts S and M respectively). A weightedcombination of the two functions yields:

xyz _(C)(λ)=α({right arrow over (XYZ)} _(S)) xyz _(S)(λ)+(1.0−α({rightarrow over (XYZ)} _(S))) xyz _(M)(λ)   (Eq. 6)

The weighting factor α(XYZ_(S)) indicates magnitude of saturation andshould approach 1 for saturated colors and approach 0 for neutralcolors. Note that for most of these approaches, two integrations willneed to be performed, one for calculating XYZ_(S) followed by a secondintegration to calculate XYZ_(C) from the above corrected human observerfunction. An example of a “reasonable” definition for α(XYZ_(S)) is:

$\begin{matrix}{{\alpha \left( {\overset{}{XYZ}}_{S} \right)} = \frac{\left\lfloor \begin{matrix}{{\max \left( {\frac{X_{S}}{X_{W}},\frac{Y_{S}}{Y_{W}},\frac{Z_{S}}{Z_{W}}} \right)} -} \\{\min \left( {\frac{X_{S}}{X_{W}},\frac{Y_{S}}{Y_{W}},\frac{Z_{S}}{Z_{W}}} \right)}\end{matrix} \right\rfloor}{\max \left( {\frac{X_{S}}{X_{W}},\frac{Y_{S}}{Y_{W}},\frac{Z_{S}}{Z_{W}}} \right)}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

where XYZ_(W) is the tristimulus XYZ value of the reference white suchas D50.

Equation 6 above can be rewritten as:

xyz _(C)(λ)= xyz _(S)(λ)+(1−α({right arrow over (XYZ)} _(S)))Δ xyz (λ)  (Eq. 8)

where Δ xyz (λ)= xyz _(M)(λ)− xyz _(S)(λ)   (Eq. 9)

2. Linear Combination of Two Sets of Integrated XYZ Results

Similarly, in the event that the color matching functions have not yetbeen defined, but corrections have been determined for the tristimulusvalues XYZ in regions of near neutral, the following XYZ-basedcorrection are used:

{right arrow over (XYZ)} _(C)=α({right arrow over (XYZ)} _(S)){rightarrow over (XYZ)} _(S)+(1.0−α({right arrow over (XYZ)} _(S))){rightarrow over (XYZ)} _(M)   (Eq. 10)

where {right arrow over (XYZ)} _(M) ={right arrow over (XYZ)} _(S)+(Y_(S) /Y _(W))Δ{right arrow over (XYZ)} _(W)   (Eq. 11)

which means Equation 10 can be rewritten as:

{right arrow over (XYZ)} _(C) ={right arrow over (XYZ)} _(S)+(1−α({rightarrow over (XYZ)} _(S)))(Y _(S) /Y _(W))Δ{right arrow over (XYZ)} _(W)  (Eq. 12)

where ΔXYZ_(W) is the correction to the XYZ value of the maximum whiteoutput of the imaging system in the vicinity of XYZ_(W). This correctionis used in lieu of using a corrected color matching function.

3. Parameterized Definition of x(λ) y(λ) z(λ) Using Non-StaticParameters

Commonly-assigned copending U.S. patent application Ser. No. 12/102,238“Improved Human Observer XYZ Functions” describes a method forparameterizing the human observer functions, and is hereby incorporatedin full. For example, one can first characterize the cone responses withthe following expression:

$\begin{matrix}{{f_{lms}\left( {\lambda,\alpha_{i},\lambda_{i},{\Delta\lambda}_{i\; 1},{\Delta\lambda}_{i\; 2},\gamma_{i\; 1},\gamma_{i\; 2},\delta_{i\; 1},\delta_{i\; 2},{\Delta\gamma}_{i\; 1},{\Delta\gamma}_{i\; 2}} \right)} = {{{{\alpha_{i}\left( {\delta_{i\; 1} + {\left( {1 - \delta_{i\; 1}} \right)^{- {({{{{\lambda - \lambda_{i}}}/2}{\Delta\lambda}_{i\; 1}})}^{({\gamma_{i\; 1} + {{\Delta\gamma}_{i\; 1}{{\lambda - \lambda_{i}}}}})}}}} \right)}\mspace{14mu} {for}\mspace{14mu} \lambda} < \lambda_{i}} = {{{\alpha_{i}\left( {\delta_{i\; 2} + \left( {1 - \delta_{i\; 2}} \right)} \right)}^{- {({{{{\lambda - \lambda_{i}}}/2}{\Delta\lambda}_{i\; 2}})}^{({\gamma_{i\; 2} + {{\Delta\gamma}_{i\; 2}{{\lambda - \lambda_{i}}}}})}}\mspace{14mu} {for}\mspace{14mu} \lambda} > \lambda_{i}}}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

where lms(1) is calculated for i=0,1,2 corresponding to l, m, srespectively. Having calculated lms(1), we can calculate xyz(1) usingthe inverse of the Hunt-Pointer-Estevez matrix M_(XYZ−>LMS):

{right arrow over (xyz)}(λ)=M _(XYZ−>LMS) ⁻¹ {right arrow over (lms)}(λ)  (Eq. 14)

All parameters can be replaced with variable coefficients as a functionof LMS:

a _(i) −>a _(i)(LMS), l _(il)−>l_(il)(LMS), etc.

Using a similar approach as before, the parameters can be optimized asdescribed in commonly-assigned copending U.S. patent application Ser.No. 12/102,238 for a set of colors generated from the saturated colormatching experiments and for Maxwell color matching experiments usingnear neutral colors. The color matching function will now be NSHOF viathe changes in the parameters, using for example,

a _(i)({right arrow over (LMS)})=α({right arrow over (LMS)})a_(iS)+(1.0−α({right arrow over (LMS)}))a _(iM)   (Eq. 15)

where

$\begin{matrix}{{\alpha \left( {\overset{}{LMS}}_{S} \right)} = \frac{\left\lfloor \begin{matrix}{{\max \left( {\frac{L_{S}}{L_{W}},\frac{M_{S}}{M_{W}},\frac{S_{S}}{S_{W}}} \right)} -} \\{\min \left( {\frac{L_{S}}{L_{W}},\frac{M_{S}}{M_{W}},\frac{S_{S}}{S_{W}}} \right)}\end{matrix} \right\rfloor}{\max \left( {\frac{L_{S}}{L_{W}},\frac{M_{S}}{M_{W}},\frac{S_{S}}{S_{W}}} \right)}} & \left( {{Eq}.\mspace{14mu} 16} \right)\end{matrix}$

where LMS_(W) is the LMS value of reference white (for example D50)converted from XYZ_(W) using the HPE matrix above.

4. Correcting XYZ via Piecewise Linear Methods

A mathematical method known as the “triple matrix” approach for appliespiecewise linear corrections in a color space that is comprised oftristimulus values XYZ or a linear transformation of XYZ. This methodhas been described in U.S. Patent Publication No. 2009/0153888 (Edge).The technique is a convenient tool for characterizing NSHOF because theinverse calculation from XYZ′−>XYZ can be easily performed.

Three matrices M_(i) for i=0,1,2 define the conversion of values XYZ tocorrected XYZ′. The requirement for the three matrices is as follows:for the X/Y region of color (X>Z and Y>Z), the conversion of XYZ values(1,0,0), (0,1,0), and (1,1,0) must be an identity transform, andconversion of “white” (1,1,1) must result in the required white pointcorrection to emulate the effect of using NSHOF, i.e. (1+a,1,1+b) where:

α=ΔX _(W) /X _(W)   (Eq. 17)

β=ΔZ _(W) /Z _(W)   (Eq. 18)

Assume Y will remain unchanged since luminosity of reference white isalways scaled to 100.0.

The similar requirements for regions Y/Z and Z/X imply the followingmatrices associated with each region:

$\begin{matrix}{M_{0} = \begin{pmatrix}1 & 0 & \alpha \\0 & 1 & 0 \\0 & 0 & {1 + \beta}\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 19} \right) \\{M_{1} = \begin{pmatrix}1 & \alpha & 0 \\0 & 1 & 0 \\0 & \beta & 1\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 20} \right) \\{M_{2} = \begin{pmatrix}{1 + \alpha} & 0 & 0 \\0 & 1 & 0 \\\beta & 0 & 1\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 21} \right)\end{matrix}$

The above requirements ensure that saturated colors will have minimalcorrection from standard XYZ whereas colors near white or neutral willbe shifted according to the XYZ correction in vicinity of white.

Having defined the M_(i) matrices for i=0,1,2, the corrected values ofXYZ are calculated as follows:

{right arrow over (XYZ)}′=M_(i){right arrow over (XYZ)}  (Eq. 22)

where

i=0 for X≧Z and Y≧Z   (Eq. 23)

i=1 for Y≧X and Z≧X   (Eq. 24)

i=2 for Z≧Y and X≧Y   (Eq. 25)

To obtain the inverse conversion, the following calculation isperformed:

{right arrow over (XYZ)}=M_(i) ⁻¹{right arrow over (XYZ)}′  (Eq. 26)

This calculation is performed for i=i_(test)=0,1,2. Next the followinglogic is applied:

if i _(test)=0 and X≧Z and Y≧Z then i=0

if i _(test)=1 and Y≧X and Z≧X then i=1

if i _(test)=2 and Z≧Y and X≧Y then i=2

Generally, only one of the three tests above will be true and i willhave a unique value with the exception of the boundaries betweenregions, in which case two tests will be true and will give identicalresults for XYZ′−>XYZ.

In a similar manner, the above can be performed in other tristimulusspaces that are linear transforms of XYZ. The above correction is validfor a device with a particular spectral characterization such as an RGBdisplay or RGB projector.

Note that Equations 19-22 can be rewritten as:

$\begin{matrix}{{\overset{\_}{XYZ}}^{\prime} = {\overset{}{XYZ} + {\Delta {{\overset{}{XYZ}}_{Corr}\left( {\overset{}{XYZ},{\Delta {\overset{}{XYZ}}_{W}}} \right)}}}} & \left( {{Eq}.\mspace{14mu} 27} \right) \\{{\Delta \; {{\overset{}{XYZ}}_{Corr}\left( {\overset{}{XYZ},{\Delta \; {\overset{}{XYZ}}_{W}}} \right)}} = {\Delta \; {M_{i}\left( {\Delta {\overset{}{XYZ}}_{W}} \right)}\overset{}{XYZ}}} & \left( {{Eq}.\mspace{14mu} 28} \right) \\{{\Delta \; M_{0}} = \begin{pmatrix}0 & 0 & \alpha \\0 & 0 & 0 \\0 & 0 & \beta\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 29} \right) \\{{\Delta \; M_{1}} = \begin{pmatrix}0 & \alpha & 0 \\0 & 0 & 0 \\0 & \beta & 0\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 30} \right) \\{M_{2} = \begin{pmatrix}\alpha & 0 & 0 \\0 & 0 & 0 \\\beta & 0 & 0\end{pmatrix}} & \left( {{Eq}.\mspace{14mu} 31} \right)\end{matrix}$

Note that α and β are functions of ΔXYZ_(W).

Example Embodiments of the Present Invention are Given Below EXAMPLE 1Method for Characterizing a Color in Terms of Tristimulus Values

The first example with a color measurement and characterization methodindicated is shown in FIG. 1. Step 110 indicates that a source of colorC is provided, which is measured and characterized. This color may be areflective material viewed in an illumination system, a region of colorrendered on a display, or a region of color projected with a digitalprojector. The spectral power distribution S(λ) of the color stimulus ismeasured at step 115 with a spectral measurement device such as aspectrophotometer controlled by a processing system.

For emissive devices, an emissive spectrophotometer is used to measureS(λ) directly. In the case of reflective material in a lightbox, it ispossible to measure S(λ) directly. However, it is more common for thespectrophotometer to illuminate the reflective material with a knowbuilt-in illumination, infer from the measurement the reflectancespectrum R(λ) of the material which is then transmitted to theprocessing system associated with the spectrophotometer. The processingsystem proceeds to obtain an estimate for the stimulus S(λ) bymultiplying the reflectance spectrum R(λ) with the SPD of theillumination in the lightbox I(λ).

Whether directly or indirectly, the SPD S(λ) of the stimulus is obtainedin step 115. In step 120 the tristimulus values XYZ are calculated, forexample using current human observer functions x(λ) y(λ) z(λ) determinedfrom the saturation method of color matching. In step 130, the values ofXYZ are used to define a new set of human observer functions x′(λ) y′(λ)z′(λ), for example using Equation 6 above.

In step 140, these revised functions x′(λ) y′(λ) z′(λ) are combined withS(λ) to generate revised tristimulus values X′Y′Z′. These values arestored in step 150 in association with the identified color C, and canbe retrieved in step 160 at a later time for the purpose of reproducingthe color on a particular media. For many applications, it is desirableto convert the tristimulus values X′Y′Z′ to a perceptually uniform colorspace such as L*a*b*.

The steps above were performed in order to correct and optimize x(λ)y(λ) z(λ) and the resulting values of XYZ. Note, however, that since thehuman observer functions are linear transforms of the cone sensitivitiesl(λ) m(λ) s(λ), this method can be performed just as well in “LMS”space, or any other three component visual color space that is based oncolor matching data.

EXAMPLE 2 Method for Matching Colors

The next example, describes a method for matching two colors indicatedin FIG. 2. A source of color C₁ which is measured and characterized atstep 210. This color may be a reflective material viewed in anillumination system, a region of color rendered on a display, or aregion of color projected with a digital projector. In step 215, thespectral power distribution S₁(λ) of the color stimulus is measured witha spectral measurement device such as a spectrophotometer controlled bya processing system as described in Example 1.

In step 220, the tristimulus values XYZ₁ are calculated, for exampleusing current human observer functions x(λ) y(λ) z(λ) determined fromthe saturation method of color matching. In step 230, the values of XYZ₁are used to define a new set of human observer functions x′₁(λ) y′₁(λ)z′₁(λ), for example using Equation 6 above, which are combined withS₁(λ) to generate revised tristimulus values X′Y′Z′₁.

In step 211 the process is repeated with a second color C₂. Step 211indicates that a source of color C₂ which is measured and characterized.This color may be a reflective material viewed in an illuminationsystem, a region of color rendered on a display, or a region of colorprojected with a digital projector.

In step 216, the spectral power distribution S₂(λ) of the color stimulusis measured with a spectral measurement device such as aspectrophotometer controlled by a processing system. In step 225 thetristimulus values XYZ₂ are calculated, for example using current humanobserver functions x(λ) y(λ) z(λ) determined from the saturation methodof color matching. In step 235, the values of XYZ₂ are used to define anew set of human observer functions x′₂(λ) y′₂(λ) z′₂(λ), for exampleusing Equation 6 above, which are combined with S₂(λ) to generaterevised tristimulus values X′Y′Z′₂. In step 240, the difference(X′Y′Z′₂−X′Y′Z′₁) between the colors is calculated. If the difference islarger than desired, color C₁ or C₂ can be adjusted in step 250 toreduce the difference to an acceptable level.

Examples of color adjustment are as follows: for a display, adjustingthe RGB value of the pixels will alter the rendering of the color. For adigital printer, adjusting pixel values CMYK will alter the printedcolor. When manufacturing paper, textiles, or other reflective material,altering the dyes and pigments results in changes to color. For sourcesof illumination such as fluorescent tubes, adjusting the quantities ofrare earth elements will affect the color of the emitted light.

The steps above were performed in order to correct and optimize x(λ)y(λ) z(λ) and the resulting values of XYZ in order to match two colorsmore effectively. Note, however, that since the human observer functionsare linear transforms of the cone sensitivities l(λ) m(λ) s(λ), thismethod can be performed just as well in “LMS” space, or any other threecomponent visual color space that is based on color matching data.

EXAMPLE 3 Method for Characterizing the Color Response of an ImagingDevice

In step 310, FIG. 3 a color is reproduced on an imaging device based oncolor coordinates C₀. For example, the device could be a display and thecolor coordinates could be a specific set of RGB values, or the devicecould be a printer and the color coordinates of the printed color couldbe CMYK.

In step 320, the spectral data of the SPD S₀(λ) of the color stimulus ismeasured. The measurement can be emissive or reflective as described inExample 1. In step 330 tristimulus values XYZ₀ are calculated using forexample, observer functions x(λ) y(λ) z(λ) that were determined usingthe saturation method of color matching.

In step 340, the values of XYZ₀ are used to define a new set of humanobserver functions x′(λ) y′(λ) z′(λ), for example using Equation 6above. In step 350, revised tristimulus values X′Y′Z′₀ are calculatedbased on the spectral data S₀(λ) and x′(λ) y′(λ) z′(λ).

In step 360, X′Y′Z′₀ are associated with color coordinates C₀. Thisprocess is repeat for a sequence of colors until an adequate list ofcolor coordinates and corresponding corrected tristimulus data isacquired. This list is then used to determine the color response of thedevice using methods that are well known in the art. The color responseis most commonly characterized in the form of a color map or color tablewhich indicates the conversion of device coordinates to predictedtristimulus values.

For many devices such as printers, it is preferable to convert thetristimulus values X′Y′Z′ to a perceptually uniform color space such asCIELAB prior to generating the characterization or map. In colormanagement literature and specifications, the XYZ or L*a*b* values arereferred to as “device independent color” values, whereas the devicecolor coordinates are referred to as “device dependent color values.”

The steps above were performed in order to correct and optimize x(λ)y(λ) z(λ) and the resulting values of XYZ in order to characterize acolor imaging device more effectively. Note, however, that since thehuman observer functions are linear transforms of the cone sensitivitiesl(λ) m(λ) s(λ), this method can be performed just as well in “LMS”space, or any other three component visual color space that is based oncolor matching data.

EXAMPLE 4 Method for Reproducing an Image on an Imaging Device

In step 410, FIG. 4, a set of spectral data S_(i)(λ) associated with aset of color device values C_(i) is measured. In step 415, a first setof tristimulous values XYZ_(i) is calculated from the spectral dataS_(i)(λ) using for example existing color matching functions x(λ) y(λ)z(λ) based on the saturation method of color matching.

In step 420, the values of XYZ_(i) are used to define a new set of humanobserver functions x′_(i)(λ) y′_(i)(λ) z′_(i)(λ), for example usingEquation 6 above. In step 430, revised tristimulus values X′Y′Z′_(i) arecalculated based on the spectral data S_(i)(λ) and x′_(i)(λ) y′_(i)(λ)z′_(i)(λ).

In step 440, a map from the device dependent values to deviceindependent values using the list of color values C_(i) is created andcorrected tristimulus values X′Y′Z′_(i). This step is typicallyaccomplished via interpolation between the list of data points or bycreating a mathematical model of the device based on the data.

In step 450, an inverse map is created from device independentcoordinates to device dependent coordinates. This is typically performedeither by inversion of the model created in step 440 or by iterativesearch methods that find the desired values of device dependentcoordinates that map to the desired device independent coordinates whenconverted via the forward map created in step 440.

In step 455, an image comprising of device independent data X′Y′Z′_(j)is provided to step 460. In step 460, the device independent data isconverted to devices coordinates C_(k) via the inverse map created instep 450. In step 470, the device coordinates C_(k) are used to createan image on the output device that has been characterized in thisprocess. Note that for many devices such as printers, it is preferableto convert the tristimulus values X′Y′Z′_(i) to a perceptually uniformcolor space such as CIELAB prior to generating the characterization ormap.

The steps above were performed in order to correct and optimize x(λ)y(λ) z(λ) and the resulting values of XYZ in order to reproduce color onan imaging device more effectively. Note, however, that since the humanobserver functions are linear transforms of the cone sensitivities l(λ)m(λ) s(λ), this mathematical description can be performed just as wellin “LMS” space, or any other three component visual color space.

EXAMPLE 5 Color Measurement Device and Digital Camera

Colorimeters approximate the sensitivity of the eye by means of filtersthat approximate x(λ) y(λ) z(λ) spectral curves. Since the NSHOF requiremore than one set of static curves, calorimeters can approximate NSHOFby using two sets of filters, one optimized for the standard humanobserver functions based on the saturation and one set optimized usingthe Maxwell method. The former can be used to estimate the tristimulusvalues XYZ_(S) and the latter for XYZ_(M) allowing the calculation forXYZ_(C) using Equations 8 and 9 above.

Likewise, cameras can utilize similar pairs of XYZ filter sets in orderto capture pixel data for both XYZ_(S) and XYZ_(M) in order to calculateand store raw XYZ_(C) data or convert the raw XYZ_(C) data to a standardcolor space such as AdobeRGB™.

FIG. 5 is a diagram of an improved colorimeter. Light stimulus emittedby computer display device 510 is measured via colorimetric measurementdevice 511. The remaining diagram indicates a magnified version of theconstruction of the measurement device. Color stimulus light passesthrough aperture 515 and strikes beamsplitter 520. Fifty percent of thelight is transmitted to detector/filter combinations 530 correspondingto human observer functions x _(S)(λ) y _(S)(λ) z _(S)(λ) derived fromcolor matching experiments using the saturation method. The other fiftypercent of the light is reflected by the beamsplitter to detector/filtercombinations 535 corresponding to human observer functions x _(M)(λ) y_(M)(λ) z _(M)(λ) derived from color matching experiments using theMaxwell method of matching whites.

Voltages V_(xs)V_(ys)V_(zs) from detector/filters 530 and voltagesV_(xM)V_(yM)V_(zM) from detector/filters 535 are converted by analog todigital converter 540 to digital values XYZ_(S) and XYZ_(M) that areindicative of the tristimulous values associated with x _(S)(λ) y_(S)(λ) z _(S)(λ) and x _(M)(λ) y _(M)(λ) z _(M)(λ). These digitalvalues are combined by processor 550 to result in corrected digitalvalues XYZ_(C) for example using Equations 7 and 10 above.

The device above contains filters optimized to simulate x(λ) y(λ) z(λ)based on the saturation method of matching and x(λ) y(λ) z(λ) based onthe Maxwell method of matching. Note, however, that since the humanobserver functions are linear transforms of the cone sensitivities l(λ)m(λ) s(λ), the filters could also be optimized to simulate l(λ) m(λ)s(λ) based on the saturation and Maxwell methods. The advantage ofdesigning such as system is that in general, three filters to simulateeach set of l(λ) m(λ) s(λ) should be adequate. By comparison, at leastfour filters are generally required to simulate each set of x(λ) y(λ)z(λ) functions.

EXAMPLE 6 Method for Measuring Colors with Colorimetric Filters

In step 610, FIG. 6, light characterized are obtained by SPD S(λ)pertaining to color C via an aperture. In step 615, filters thatsimulate x _(S)(λ) y _(S)(λ) z _(S)(λ) and x _(M)(λ) y _(M)(λ) z _(M)(λ)which are placed in front of light detectors for obtaining signalsproportional to the intensity of light are provided. In step 620,voltages V_(xs)V_(ys)V_(zs) and voltages V_(xM)V_(yM)V_(zM) are obtainedfrom detectors placed behind the filters provided in step 615. In step630, these voltages are converted by analog to digital converters todigital values XYZ_(S) and XYZ_(M) that are indicative of thetristimulous values associated with x _(S)(λ) y _(S)(λ) z _(S)(λ) and x_(M)(λ) y _(M)(λ) z _(M)(λ).

In step 640, these digital values are combined by a processor to resultin corrected digital values X′Y′Z′ for example using Equations 7 and 10above. In step 650, the values X′Y′Z′ are stored and associated withcolor C. In step 660, the corrected values X′Y′Z′ are retrieved whenreproducing color C on a different device.

The method above utilizes filters optimized to simulate x(λ) y(λ) z(λ)based on the saturation method of matching and x(λ) y(λ) z(λ) based onthe Maxwell method of matching. Note, however, that since the humanobserver functions are linear transforms of the cone sensitivities l(λ)m(λ) s(λ), the filters could also be optimized to simulate l(λ) m(λ)s(λ) based on the saturation and Maxwell methods. The advantage ofdesigning such as system is that in general, three filters to simulateeach set of l(λ) m(λ) s(λ) should be adequate. By comparison, at leastfour filters are generally required to simulate each set of x(λ) y(λ)z(λ) functions.

EXAMPLE 7 Method for Matching Colors Between Two Systems

This example, describes a method for matching two colors between twosystems wherein the human observer function for the Maxwell method hasnot been determined. Assume that the standard observer functions whichare based on the saturation method of matching are known, and assumethat both systems can be used to render a white color for empiricallyobtaining a white correction in units of XYZ.

For example, one system could be a lightbox with a neutral white paperpresented for viewing, while the second system could be a display whichis rendering a simulation of that paper via color management. Assume aswell that the display is adjusted with regard to the white in order toensure that the calculated values of tristimulus XYZ from the measuredS(λ) from the white of each system are very similar. Also assume thatthe white on the display can be adjusted in the event that there is avisual mismatch between the whites of the two systems, and that themeasured difference in XYZ can be obtained after the two whites havebeen confirmed to match visually.

Step 710, FIG. 7, wherein the two whites of the two systems is comparedand confirm via measurement that XYZ_(W1)=XYZ_(W2) for System 1 andSystem 2 using the conventional saturation-based observer functions. Instep 720, the white in System 2 is adjusted until the whites of the twosystems match and measure the difference in XYZ between the two whites,ΔXYZ_(W).

In steps 730 and 740, spectral data S₁(λ) for color C₁ of System 1 andspectral data S₂(λ) for color C₂ of System 2 is measured. In steps 735and 745, tristimulous values XYZ₁ and XYZ₂ is calculated. In step 750,calculate corrected values

X′Y′Z′ ₂ =XYZ ₂ +ΔXYZ _(corr)(XYZ ₂ ,ΔXYZ _(W))   (Eq. 32)

wherein the correction ΔXYZ_(corr) is a function of the tristimulusvalues of the second color (XYZ₂) and the tristimulus values of thewhite correction, ΔXYZ_(W), determined in step 720. This calculation isbased on Equation 27 above.

In step 760, the difference ΔXYZ between X′Y′Z′₂ and XYZ₁ is determined.If the difference exceeds a predetermined threshold of acceptability,colors C₁ or C₂ can be adjusted in step 770 to reduce the difference. Inthe example of colors viewed in a lightbox compared to colors renderedon a display, it is far easier to adjust the colors on the display toreduce the magnitude of ΔXYZ.

The steps above were performed in order to correct and optimize XYZ inorder to improve color matching for a particular pair of systems. Note,however, that since the tristimulus values XYZ are linear transforms ofthe cone LMS values, this mathematical description can be performed justas well in “LMS” space, or any other three component visual color spacewhose tristimulus values are derived from color matching data.

One option for ease of continuity with existing color management systemsis to use current systems as the starting point using currenttristimulus XYZ values and profiles and then applying calculatedcorrections to standard values on a per device or device plusillumination basis. This approach has been implemented as follows.Piecewise linear correction similar to that described above was appliedto an AdobeRGB linear color space. The value of ΔXYZ_(W) was calculatedfrom the value of ΔLab for each combination of display and lightingsystem. An initial display was used as a starting point. A correction ofΔa* and Δb* was determined visually for the display in order to matchthe white/gray appearance of black and white images. A unique pair ofΔa* and Δb* values was determined for that display for each type ofillumination. In particular, unique corrections for GTI D50 lighting andJustNormlicht D50 lighting were determined. Since the normalized valueof L*a*b* for D50 illumination or white point is L*a*b*=(100,0,0), thevalues of Δa* and Δb* were added to this “perfect white” in order tocalculate an adjusted white (100, Δa*, Δb*). The values of (100, Δa*,Δb*) and (100,0,0) were converted to XYZ and the difference of theresult defined the values of ΔXYZ_(W) for use in the piecewise methoddescribed above.

Next, new monitors were added to the system. For each monitor, values(Δa*_(m), Δb*_(m)) were determined to address differences betweenmonitors due to NSHOF effects. These corrections were added to create anew corrected white value (100, Δa*+Δa*_(m), Δb*+Δb*_(m)) in order tocalculate the unique values of ΔXYZ_(W) for each display+illuminationcombination in the system. All the above corrections were obtainedvisually by adjusting the display until black and white images on thedisplay demonstrated similar white/gray balance to corresponding blackand white digital prints placed in the illumination system.

Applying and/or Embedding NSHOF in Displays and Projection Systems

In order to apply NSHOF to displays and projectors, a source image withassociated profile (either a device profile or standard color space suchas AdobeRGB™) is converted to PCS in which the PCS is calculated usingone of the NSHOF methods above from spectral data or modifiedcolorimeter. The image data is then converted from the PCS to thedestination RGB system by means of a profile associated with the displayor projection system calculated using PCS values determined using NSHOF.

Alternatively, the conversion from RGB to PCS to RGB can be embeddedwithin the imaging system. Thus, a standard RGB space can be used fordigital images. The standard RGB space is then assumed within thedisplay system and the conversion from standard RGB to output RGBperformed internally. If the NSHOF are used as the means for definingthe PCS within the imaging device, such conversion would be equivalentto performing it within a color management system.

Certifying illumination Sources with NSHOF

Illumination sources such as professional D50 lighting for the graphicarts have several requirements:

-   -   a) Acceptable match to theoretical D50 according to XYZ        calculations,    -   b) Acceptable absolute match of XYZ values for illuminated        colored samples when calculated using the illumination and D50,        and    -   c) Acceptable relative XYZ match between ink jet and printing        ink samples which have been adjusted to produce identical XYZ        values under D50.

Despite these attempts to achieve similarity between differentmanufactured D50 light sources, in practice they differ by several ΔE inwhite balance. This can be confirmed by adjusting a displayed image inorder to match first one and then another light source from twodifferent venders.

The above procedure for optimizing the raw combinations of rare earthelements in order to achieve the above specification can be modified touse the NSHOF in order to calculate and compare XYZ between the lightsource and D50. Doing so will result in a very good match between thewhite balance. If the corresponding difference measured and calculatedin this manner is 1-2 ΔE, the visual similarity will be almostidentical. Based on the observations of the studies performed byThornton, the error corrected by NSHOF for white light sources could beas large as 20 ΔE for combinations of very different sets of narrow bandlight sources.

Certifying Papers, Paints Textiles with NSHOF

One of the rationales for repeating the color matching experiments(performed by Stiles and Burch) was unacceptable matching of paperwhites. Since the experiment was performed using the saturation methodrather than the Maxwell method, there did not appear to be confirmationfrom the paper industry that the new effort had succeeded. For the paperindustry, a full averaged study of color matching functions using theMaxwell method is needed. This result can be combined with existingsaturation based observer functions to create NSHOF that should succeedvery well in matching paper whites visually when the corrected XYZ's arecalculated from spectral measurements. Equations 6-9 would be effectivefor offering a single standard of measurement to the paper industry thatwould be valid for measurement of all media such as paints, textiles,etc.

The NSHOF described above should be effective for specifying and forrendering color in all industries where critical color control andreproduction is required.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   110 provide source of color C-   115 measure spectral values S(λ) for color stimulus associated with    color C-   120 calculate tristimulus values XYZ-   130 define new human observer functions x′(λ) y′(λ) z′(λ) based on    tristimulus values XYZ-   140 calculate new X′Y′Z′_(I) based on S(λ) and new human observer    functions x′(λ) y′(λ) z′(λ)-   150 store X′Y′Z′ and associate with color C from media above-   160 retrieve X′Y′Z′ when reproducing color C from media above-   210 provide source of color C₁-   215 measure spectral values S₁(λ) for color stimulus associated with    color C₁-   211 Provide source of color C₂-   216 measure spectral values S₂(λ) for color stimulus associated with    color C₂-   220 calculate tristimulus values XYZ₁-   225 calculate tristimulus values XYZ₂-   230 calculate X′Y′Z′₁ using human observer functions x′₁(λ) y′₁(λ)    z′₁(λ) defined by values XYZ₁-   235 calculate X′Y′Z′₂ using human observer functions x′₂(λ) y′₂(λ)    z′₂(λ) defined by values XYZ₂-   240 calculate difference (X′Y′Z′₂−X′Y′Z′₁)-   250 adjust color C₁ or C₂ to reduce difference-   310 reproduce color on imaging device based on device color    coordinates C₀-   320 measure spectral data S₀(λ)-   330 calculate tristimulus values XYZ₀-   340 define new human observer functions x′(λ) y′(λ) z′(λ) based on    XYZ₀-   350 calculate new X′Y′Z′₀ based on S₀(λ) and new human observer    functions x′(λ) y′(λ) z′(λ)-   360 associate X′Y′Z′₀ with color C₀-   410 measure set of spectral data S_(i)(λ) for a set of color device    values C_(i)-   415 calculate tristimulus values XYZ_(i)-   420 define x′_(i)(λ)y′_(i)(λ) z′_(i)(λ) as a function of values    XYZ_(i)-   430 calculate X′Y′Z′_(i) from S_(i)(λ) and x′_(i)(λ)y′_(i)(λ)    z′_(i)(λ)-   440 create map from device dependent values to device independent    values-   450 create map from device-independent values to device-dependent    values-   455 provide device independent image data X′Y′Z′_(j)-   460 convert device independent image data X′Y′Z′_(j) to device color    values C_(k)-   470 reproduce image on color device using device color values C_(k)-   510 computer display device-   511 colorimetric measurement device-   515 light aperture-   520 beamsplitter-   530 filters and light detectors pertaining to x _(S)(λ) y _(S)(λ) z    _(S)(λ)-   535 filters and light detectors pertaining to x _(M)(λ) y _(M)(λ) z    _(M)(λ)-   540 A/D converter for converting voltages to XYZ_(S) and XYZ_(M)-   550 processor for combining XYZ_(S) and XYZ_(M) to calculate XYZ_(C)-   610 obtain light S(λ) via an aperture for a color C-   615 provide filters that simulate x_(S)(λ) y_(S)(λ) z_(S)(λ) and    x_(M)(λ) y_(M)(λ) z_(M)(λ)-   620 obtain signals V_(xs)V_(ys)V_(zs) and V_(xM)V_(yM)V_(zM)    proportional to the intensity of light through filters via light    detectors-   630 convert signals to digital values XYZ_(S) and XYZ_(M) Via A/D    converters-   640 combine αXYZ_(S)+(1−α)XYZ_(M) to obtain X′Y′Z′ where    α=α(XYZ_(S))-   650 store X′Y′Z′ and associate with color C-   660 retrieve X′Y′Z′ when reproducing color C from media above-   710 compare white colors W₁ and W₂ with tristimulus values    XYZ_(W1)=XYZ_(W2) in Systems 1 and 2-   720 empirically determine correction ΔXYZ_(W) to XYZ_(W2) in order    to achieve visually matching whites W₁ and W₂-   730 measure spectral data S₁(λ) for color C₁-   735 calculate XYZ₁-   740 measure spectral data S₂(λ) for color C₂-   745 calculate XYZ₂-   750 calculate X′Y′Z′₂=XYZ₂+ΔXYZ_(corr) (XYZ₂, ΔXYZ_(W))-   760 calculate difference ΔXYZ=X′Y′Z′₂−XYZ₁-   770 adjust C₁ or C₂ to reduce ΔXYZ

1. A method for matching colors comprising: providing a first source ofa first color and a second source of a second color; measuring a firstset of spectral values for first color stimuli associated with the firstcolor with a spectral measurement device controlled by a digitalprocessing system; calculating a first set of tristimulus values fromthe first set of spectral values; defining a first set of human observercolor matching functions, the first set of human observer color matchingfunctions being functions of the first set of tristimulus values of thefirst color; calculating a first set of corrected tristimulus valuesfrom the first set of spectral values using the first set of humanobserver color matching functions; measuring a second set of spectralvalues for second color stimuli associated with the second color withthe spectral measurement device controlled by the digital processingsystem; calculating a second set of tristimulus values from the secondset of spectral values; defining a second set of human observer colormatching functions, the second set of human observer color matchingfunctions being functions of the second set of tristimulus values of thesecond color; calculating a second set of corrected tristimulus valuesfrom the second set of spectral values using the second set of humanobserver color matching functions; determining a difference between thefirst set of corrected tristimulus values and the second set ofcorrected tristimulus values; and adjusting one of the first and thesecond source to reduce the difference.
 2. The method of claim 1,wherein the determining a difference is performed after converting thefirst and second set of the tristimulus values to a perceptually uniformcolor space.
 3. The method of claim 1, wherein at least one of the firstsource and the second source is an illumination system.
 4. The method ofclaim 2, wherein at least one of the first source and the second sourceis an illumination system.
 5. The method of claim 1, wherein at leastone of the first source and the second source is a reflective materialviewed in an illumination system.
 6. The method of claim 2, wherein atleast one of the first source and the second source is a reflectivematerial viewed in an illumination system.
 7. The method of claim 1,wherein at least one of the first source and the second source is anemissive display system.
 8. The method of claim 2, wherein at least oneof the first source and the second source is an emissive display system.9. The method of claim 1, wherein at least one of the first source andthe second source is a digital projection system.
 10. The method ofclaim 2, wherein at least one of the first source and the second sourceis a digital projection system.
 11. The method of claim 1, wherein thefirst and second set of human observer color matching functions aredefined according to the equation xyz _(C)(λ)=α({right arrow over(XYZ)}_(S)) xyz _(S)(λ)+(1.0−α({right arrow over (XYZ)}_(S))) xyz_(M)(λ) wherein xyz _(M)(λ) denotes human observer functions obtainedusing the Maxwell method of matching whites, xyz _(S)(λ) denotes humanobserver functions derived from the saturation method, xyz _(C)(λ)refers to the corrected human observer function, and α({right arrow over(XYZ)}_(S)) is a function of tristimulus values calculated from xyz_(S)(λ).
 12. The method of claim 1, wherein the tristimulus values arethe cone values LMS and the color matching functions are the coneresponses l(λ) m(λ) s(λ).